Group III nitride compound semiconductor device

ABSTRACT

The present invention provides a Group III nitride compound semiconductor device in which the amount of a current allowed to be applied on a p-type pad electrode can be increased. 
     That is, in the Group III nitride compound semiconductor device according to the present invention, a portion of a translucent electrode coming in contact with a circumferential surface of the p-type pad electrode is formed as a thick port ion to thereby increase the area of contact between the circumferential surface and the translucent electrode to thereby increase the current allowed to be applied on the p-type pad electrode. In addition, the use of the thick portion prevents cracking from occuring between the translucent electrode and the circumferential surface of the pad electrode.

TECHNICAL FIELD

The present invention relates to a Group III nitride compoundsemiconductor device. More specifically, it relates to improvement in ap-type translucent electrode of a Group III nitride compoundsemiconductor device.

BACKGROUND ART

In a Group III nitride compound semiconductor light-emitting device suchas a blue light-emitting diode, especially, in a light-emitting devicehaving a substrate disposed on a lower side, a thin translucentelectrode is formed on an almost whole surface of a p-type contact layerhaving relatively large resistance in order to distribute a current intothe p-type contact layer evenly to thereby obtain uniform light emissionform the whole area of a light-emitting layer. Because wire-bondingcannot be applied directly to such a thin translucent electrode, a padelectrode is formed thereon (see Japanese Unexamined Patent PublicationNo. JP-A-H09-320984, and so on).

The present inventors have made an examination of further reduction inthickness of the translucent electrode to enhance light transmittance ofthe translucent electrode. As a result, the following problem to besolved has been found.

That is, as described in the aforementioned patent publications, it isnecessary to apply a heat treatment in order to secure ohmic contactbetween the translucent electrode and the p-type contact layer, securetranslucency of the translucent electrode and further secure adhesion ofthe pad electrode to the translucent electrode and the p-type contactlayer. It is conceived that when the heat treatment is carried out, thematerial of the pad electrode and the material of the translucentelectrode are alloyed so that the pad electrode and the translucentelectrode are substantially integrated with each other. Hence, acircumferential surface of the pad electrode becomes a surface connectedto the translucent electrode.

Incidentally, the p-type contact layer made of a Group III nitridecompound semiconductor is different in linear expansion coefficient offormation material from the translucent electrode/pad electrode (p-typeelectrode structure) integrated with each other by the heat treatment.Because the linear expansion coefficient of the latter made of metal ishigher than that of the former, tensile stress occurs in the p-typeelectrode structure after the heat treatment. For this reason, there isa possibility that the interface between the circumferential surface ofthe pad electrode and the translucent electrode may crack because thisportion is inferior in mechanical strength. Because it is conceived thatthe current allowed to be applied to the pad electrode (allowablecurrent) is proportional to the area of contact between the padelectrode and the translucent electrode, the allowable current isreduced if there is such a crack. While both greater reduction in chipsize and improvement in efficiency of light emission to the outside arerequired, reduction in size of the pad electrode is also required. Onthe other hand, maintenance of the status quo of the allowable currentor increase of the allowable current is required. It is howeverimpossible to satisfy these requirements if there is such a crack. Thisis because the area of contact between the pad electrode and thetranslucent electrode is not sufficient.

Moreover, because the area of contact between the translucent electrodeand the circumferential surface of the pad electrode is reduced in thecase were reduction in thickness of the translucent electrode advanced,this point is also a factor to cause limitation in the allowablecurrent.

In order to examine the life and durability, a high current may need tobe applied to the light-emitting device under a high-temperatureenvironment. In this case, if there is a crack between thecircumferential surface of the pad electrode and the translucentelectrode, the allowable current is limited as described above. There istherefore a Possibility that originally required stringent examinationcannot be executed because the current allowed to be applied at the timeof examination is limited. In addition, there may be a disadvantage thatburning or the like occurs in the cracking portion at the time ofexecution of examination so that durability of other members of thelight-emitting device cannot be substantially examined any more. Thismeans that long-term durability is poor in the case where a high currentis applied under a high-temperature environment such as the outdoors.

DISCLOSURE OF THE INVENTION

As described above, the linear expansion coefficient difference betweenthe p-type contact layer and the p-type electrode structure constitutedby a combination of the pad electrode and the translucent electrodecauses cracking in the interface between the circumferential surface ofthe pad electrode and the translucent electrode. As a result, the areaof contact between the two is reduced to thereby bring about theaforementioned various problems. The invention is to solve theaforementioned problems found newly by the inventors. The configurationof the invention is as follows. That is, a Group III nitride compoundsemiconductor device has a translucent, electrode including a portionwhich comes in contact with a circumferential surface of a p-type padelectrode and which is formed as a thick portion.

According to the Group III nitride compound semiconductor deviceconfigured in this manner, the portion of the translucent electrodecoming in contact with the circumferential surface of the p-type padelectrode is made thick, so that the area of a joint surface between thetranslucent electrode and the circumferential surface of the padelectrode is increased, compared with the translucent electrode havingno thick portion. Because it is inferred that the allowable current isproportional to the area of the joint surface between the translucentelectrode and the circumferential surface of the pad electrode, thecurrent allowed to be applied to the pad electrode is increased in thedevice according to the present invention in which the thick portion isprovided.

In addition, because adhesion (mechanical strength) between thetranslucent electrode and the circumferential surface of the padelectrode is strengthened by the presence of the thick portion, anycrack hardly occurs between the two. Also from this point of view, theallowable current is prevented from being limited.

From another aspect, the invention can be also grasped as follows. Thatis,

a translucent electrode in a Group III nitride compound semiconductordevice wherein a portion which comes in contact with a circumferentialsurface of a p-type pad electrode is formed as a thick portion.

The configuration of the invention grasped from a further aspect is asfollows. That is,

a p-type electrode structure constituted by

a translucent electrode and

a pad electrode formed on a p-type semiconductor layer in a Group IIInitride compound semiconductor device, has a feature in that a portionof the translucent electrode coming in contact with a circumferentialsurface of the pad electrode is formed as a thick portion.

The configuration of the invention grasped from an aspect of a producingmethod is as follows. That is, a method of producing a Group III nitridecompound semiconductor device, has the steps of: forming a translucentelectrode material layer on a p-type semiconductor layer; forming athick portion material layer on the translucent electrode materiallayer; and forming a p-type pad electrode material layer smaller indiameter than the thick portion material layer on the thick portionmaterial layer.

Further, the configuration of the invention grasped from another aspectas as follows. That is,

a method of producing a p-type electrode structure in a Group IIInitride compound semiconductor device, has the steps of: forming a thickportion material layer on a translucent electrode material layer; andforming a p-type pad electrode material layer smaller in diameter thanthe thick portion material layer on the thick portion material layer

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and 1B show a method for producing a p-type electrode structureas an embodiment of the invention;

FIG. 2A is a plan view showing an embodiment of the p-type electrodestructure;

FIG. 2B is a plan view showing another embodiment of the p-typeelectrode structure;

FIGS. 3A and 3B show a method for producing a p-type electrode structureas another embodiment;

FIG. 4 is a plan view showing a p-type pad electrode as anotherembodiment;

FIG. 5 is a plan view showing a p-type pad electrode as a furtherembodiment;

FIGS. 6A to 6D show the configurations of an indented portion of ap-type pad electrode as still further embodiments; and

FIG. 7 shows a light-emitting diode as an embodiment of the invention.

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION

Members of the invention will be described below in detail.

As shown in FIG. 1B, a translucent electrode 11 has a thick portion 13connected to a circumferential surface of a pad electrode 15, and a thinportion 12 as the remainder.

The thickness and material of the thin portion 12 are not particularlylimited but it is preferable that the thin portion 12 is made of atranslucent material such as a gold alloy and is made as thin aspossible from the point of view of transmitting light in the case of alight-emitting device. For example, in the case of a gold alloy, thethickness of the thin portion 12 is preferably set to be in a range offrom 4 to 40 nm, more preferably in a range of from 4 to 30 nm, furtherpreferably in a range of from 4 to 15 nm, most preferably in a range offront 4 to 10 nm.

The thickness of a thickest portion (a portion joined to the padelectrode 15) of the thick portion 13 is preferably set to be in a rangeof from 1.5 times to 15 times, more preferably in a range of from 1.5times to 10 times, further preferably in a range of from twice to 8times, most preferably in a range of from twice to 6 times as large asthe thickness of the thin portion 13.

The material of the thick portion 13 is not particularly limited but itis preferable that the thick portion 13 is made of the same kind ofmaterial as the material of the pad electrode in order to ensure strongadhesion to the pad electrode. Similarly, because the thick portion 13needs strong adhesion to the thin portion 13, it is preferable that thematerial of the thick portion 13 is made of the similar kind of materialas the material of the thin portion 12. Specifically, when the thinportion 12 is made of a gold alloy and the seal electrode 15 is alsomade of a gold alloy, the thick portion 13 is similarly made of gold ora gold alloy. Incidentally, in this embodiment, both the thick portion13 and the thin portion 12 are made of a cobalt-gold alloy.

The shape of the pad electrode 15 is not particularly limited if anelectrically conductive wire can be bonded onto a top portion of the padelectrode 15. Incidentally, when an indented portion is formed in acircumferential surface of the pad electrode 15, a large allowablecurrent is obtained.

The indented portion of the pad electrode will be described below.

In this case, as shown in FIG. 4, the pad electrode 45 is constituted bya combination of a base portion 46 and an indented portion 47 around thebase portion 46. An electrically conductive wire is bonded to the baseportion 46. In the drawing, the dotted line is given to designate theboundary between the bass portion 46 and the indented portion 47 but theboundary between the two cannot be decided clearly. For example, on theassumption that the base portion 46 is defined on the basis of a virtualline (chain double-dashed line) connecting outward end portions of theindented portion 47 to one another, provision of concave portions in thebase portion may be regarded as resulting in formation of the indentedportion in the circumferential surface of the pad electrode 45. Inshort, the shape of the base portion 46 is not particularly limited ifthe base portion 46 can have an area sufficient to bond an electricallyconductive wire. As shown in FIG. 5, it may be formed as a square baseportion 56. Any other shape such as a polygonal shape or an ellipticshape may be also used.

The shape of the indented portion 47 on a side of the pad electrode 45is defined on the basis of a mask pattern of the pad electrode. Hence,the indented portion 47 is serrated in a direction of circumference ofthe pad electrode 45 but is not indented in a direction of height of thepad electrode 45.

The indented portion 47 in the circumferential surface of the padelectrode 45 is provided for maximizing the area of the circumferentialsurface to thereby increase the area of contact between the padelectrode and the translucent electrode. Accordingly, the presentinvention is different in purpose and operation from inventions (e.g.,Japanese Unexamined Patent Publication No. JP-A-H08-340131, JapaneseUnexamined Patent Publication No. JP-A-H10-275934, Japanese UnexaminedPatent Publication No. JP-A-H10-117017, etc.) which intends to applycurrents to the p-type semiconductor layer evenly through an auxiliaryelectrode extended from the case portion 46 of the pad electrode.

The shape of such serration can be selected optionally. FIG. 4 shows acircle approximated on the basis of a lattice. When, for example, acircle with the radius of 100 μm is approximated on the basis of alattice with a patch of 3 μm, the length of the circumference of thecircle is equal to 100 μm×4. On the other hand, the length of a smoothcircle with the radius of 100 μm is equal to 100 μm×3.14. The former islonger by about 27%. Accordingly, even in the case where cracks occur inthe interface between the pad electrode and the translucent electrode,the area of contact between the pad electrode and the translucentelectrode is enlarged by the enlargement of the interface if the rate ofcracks per unit length of the interface is constant. Hence, in simplecalculation, the allowable current in the pad electrode according to thepresent invention is increased by 27%, compared with that in theconventional circular pad electrode. In other words, even in the casewhere the diameter of the pad electrode according to the presentinvention is reduced by 27%, the same allowable current as that in theconventional pad electrode can be ensured.

In an example shown in FIG. 5, an indented portion 57 shaped like asquare in plan view is provided. In this case, the area of thecircumferential surface of the pan electrode 55 is about twice as largeas the area of the circumferential surface of the pad electrode havingno indented portion 57. Hence, the area of contact with the translucentelectrode increases to about twice, so that the allowable currentincreases to about twice on calculation.

FIGS. 6A to 6D show other examples of the shape of the indented portionformed in the circumferential surface of the pad electrode. The exampleof the indented portion 61 shown in FIG. 6A is serrated like a saw-teethshape. The example of the indented portion 62 shown in FIG. 6B is a sinecurve. The examples of the indented portions 63 and 64 shown in FIGS. 6Cand 6D are hook shapes.

The indented portion is provided for maximizing the area of contactbetween the pad electrode and the translucent electrode. However, if theamplitude of concavity and convexity is too large, that is, ifprotrusions are too large, light transmitted through the translucentelectrode is shaded by the protrusions undesirably. Therefore, theamplitude of concavity and convexity, that is, the difference betweenthe distance L1 from the center of the pad electrode to a forward end ofa protrusion and the distance L2 from the center of the pad electrode toa bottom of a recess adjacent to the protrusion, is preferably set to bein a range of from 2 so 40 μm, more preferably in a range of from 2 to30 μm, further preferably in a range of from 2 to 15 μm, most preferablyin a range of from 3 to 10 μm.

Such an indented portion as preferably formed in the circumferentialsurface of the pad electrode may be also preferably formed in the outercircumferential surface of each of a thick portion material layer 3shown in FIG. 1A and a thick portion material layer 33 shown in FIG. 3Afor The same purpose and operation. The design concept of the indentedportion to be provided in the thick portion material layer is the sameas that for the pad electrode.

The circumferential surface of the pad electrode is preferably inclined.That is, the pad electrode 45 (FIG. 4) is shaped like a truncated coneas a whole, and the pad electrode 55 (FIG. 5) is shaped like a truncatedquadrangular pyramid as a whole. When the circumferential surface of thepad electrode is tapered, a protective film (SiO₂ film) formed onsurfaces of the pad electrode and the translucent electrode can be alsoformed on the tapered portion so that the protective film substantiallyhas such a film thickness as designed.

The material for forming the thin portion occupying a main portion ofthe translucent electrode 11 is not particularly limited. In the case ofa light-emitting device, as shown in FIG. 1A, the translucent electrode11 is preferably formed by the steps of: sequentially laminating a Colayer as a first electrode layer and an Au layer as a second electrodelayer or the lower side to thereby form a translucent electrode materiallayer 1; and heating the translucent electrode material layer 1 to alloyCo and Au.

It is preferable that the constituent element of the first electrodelayer is an element lower in ionization potential than the constituentelement of the second electrode layer, and that the constituent elementof the second electrode layer is set as an element more excellent inohmic property with semiconductor than the constituent element of thefirst electrode layer. A heat treatment is also applied to the electrodelayers in order to alloy the electrode layers with the p-type contactlayer. By the heat treatment, the element distribution viewed in adepthwise direction from a surface of the semiconductor becomes adistribution formed so that the constituent element of the secondelectrode layer penetrates more deeply than the constituent element ofthe first electrode layer. That is, the element distribution of theelectrode layers is reverse to the distribution at the time of formationof the electrode layers. After the formation of the electrode layers,the constituent element of the second electrode layer formed on theupper side is located on the lower side whereas the constituent elementof the first electrode layer formed on the lower side is located on theupper side.

Preferably, the constituent element of the first electrode layer is atleast one kind of element selected from the group consisting of nickel(Ni), cobalt (Co), iron (Fe), copper (Cu), chromium (Cr), tantalum (Ta),vanadium (V) maganese (Mn), aluminum (Al), and silver (Ag). Thethickness of the first electrode layer is set to be in a range of from0.5 to 15 nm. The constituent element of the second electrode layer isat least one kind of element selected from the group consisting ofpalladium (Pd), gold (Au), iridium (Ir), and platinum (Pt). Thethickness of the second electrode layer is set to be in a range of from3.5 to 25 nm. More especially, the constituent element of the firstelectrode layer is Co or Ni whereas the constituent element of thesecond electrode layer is Au. In this case, the element distributionviewed in a depthwise direction from a surface of the semiconductorbecomes a distribution formed by the heat treatment so that Aupenetrates more deeply than Co or Ni.

Most especially, the first electrode layer is made of Co.

The material for forming the pad electrode is not particularly limitedtoo. In the case of application to a light-emitting device, for example,as shown in FIG. 1A, the pad electrode 15 is preferably formed by thesteps of: sequentially laminating a V layer as a first metal layer, anAu layer as a second metal layer and an Al layer as a third metal layeron the lower side to form a pad electrode material layer 5; and heatingthe pad electrode material layer 5.

The first metal layer is made of an element lower in ionizationpotential than that of the second metal layer so that the first metallayer can be firmly bonded to a layer thereunder. It is preferable thatthe second metal layer is made of an element excellent in bondingcharacteristic to Al or Au and not reactive to the translucent electrodewhereas the third metal layer is made of an element firmly bonded to theprotective film.

Preferably, the constituent element of the first metal layer is at leastone kind of element selected from the group consisting of nickel (Ni),iron (Fe), copper (Cu), chromium (Cr), tantalum (Ta), vanadium (V),manganese (Mn), aluminum (Al), silver (Ag), and cobalt (Co). Thethickness of the first metal layer is in a range of from 5 to 300 nm.

Preferably, the constituent element of the third metal layer is at leastone kind of element selected from the group consisting of aluminum (Al),nickel (Ni), and titanium (Ti) The thickness of the third metal layer isin a range of from 1 to 30 nm.

Preferably, the constituent element of the second metal layer is gold(Au). The thickness of the second metal layer is in a range of from 300to 5000 nm.

As shown in FIG. 1A, the thick portion 13 is formed when the ring-likethick portion material layer 3 formed on the translucent electrodematerial layer 1 is heated. The material nor forming the thick portionmaterial layer 3 is not particularly limited if it can obtain sufficientadhesion to the material of the pad electrode 15 and to the material ofthe thin portion 12 by heating.

For example, the thick portion material layer 3 can be formed as acombination of a first layer and a second layer. The first layer is madeof at least one kind of element selected from the group consisting ofnickel (Ni), cobalt (Co), iron (Fe), copper (Cu), chromium (Cr),tantalum (Ta), vanadium (V), manganese (Mn), aluminum (Al), and silver(Ag). The second layer is made of at least one kind of element selectedfrom the group consisting of palladium (Pd), gold (Au), iridium (Ir),and platinum (Pt).

In the case of a light-emitting device, because both the translucentelectrode and the pad electrode are often made of a gold alloy, thethick portion material layer 3 is preferably made of a gold alloy. Thethick portion material layer 3, however, may be provided as the firstlayer made of one kind of element selected from the aforementioned thickportion materials if the thick portion material later 3 can obtainsufficient adhesion to the then portion 12 and to the pad electrode 15.Although this embodiment has shown the case where the thick portionmaterial layer 3 is made of the same kind of cobalt-gold alloy as thatof the translucent electrode material layer 1, the thick portionmaterial layer 3 may be made of only gold.

The diameter of the thick portion material layer 3 is set as a diameterlarger than the diameter of a lower edge of the pad electrode materiallayer 1, so chat a portion forced out of the lower edge is formed as thethick portion of the translucent electrode 11.

Hence, the thickness of the thick portion material layer is designed inaccordance with the required thickness of the thick portion 13. Morespecifically, the thickness of the thick portion material layer 3 ispreferably set to be in a range of 0.5 times to 14 times, morepreferably in a range of from 0.5 to 9 times, further preferably in arange of from 1 to 7 times, most preferably in a range of from 1 to 5times as large as the thickness of the translucent electrode materiallayer 1.

The outer size of the thick portion material layer 3 is also designed inaccordance with the required width of the thick portion 13. The width Wof a protruded portion of the thick portion material layer 3 which isgoes out from the lower edge of the pad electrode material layer 1 ispreferably set to be in a range of from 1 to 30 μm, more preferably in arange of from 2 to 20 μm, further preferably in a range of from 2 to 15μm, most preferably in a range of from 3 to 10 μm. Because the thickportion 13 is inferior in light transmission characteristic, there is apossibility that light-emitting efficiency of the light-emitting devicemay be reduced if the width W is larger than 30 μm. in addition, asshown in FIG. 1A, the thick portion 13 is provided for smoothlyconnecting the circumferential surface of the pad electrode 15 and thethin portion 12 of the translucent electrode 11 to each other byheating. If the width W is smaller than 1 μm, there is a possibilitythat mechanical strength of the interface between the thin portion 12and the thick portion 13 becomes insufficient because the joint betweenthe two is precipitous.

It is preferable that the width W is uniform on the whole circumferenceof the pad electrode material layer 1. For this reason, it is preferablethat the center of the thick portion material layer 3 and the center ofthe pad electrode material layer 5 coincide with each other as shown inFIG. 2A. FIG. 2B shows an example of a square pad electrode materiallayer 25. In the drawing, the reference numeral 23 designates a thickportion material layer.

Further, as shown in FIG. 3A, a flat plate-like thick portion materiallayer 33 may be also used. A pad electrode material layer 35 is formedon the thick portion material layer 33 so that the centers of the twocoincide with each other.

The translucent electrode material layer 1, the thick portion materiallayer 3 and the pad electrode material layer 5 are alloyed by the heattreatment.

The heat treatment is preferably carried out in the presence of gascontaining oxygen. At this title, as the oxygen-containing gas, it ispossible to use at least one member or a mixture gas selected from thegroup consisting of O₂, O₃, CO, CO₂, NO, N₂O, NO₂, and H₂O.Alternatively, it is possible to use a mixture gas of inert gas and atleast one member selected from the group consisting of O₂, O₃, CO, CO₂,NO, N₂O, NO₂, and H₂O. Alternatively, it is possible to use a mixturegas of inert gas and a mixture gas selected from the group consisting ofO₂, O₃, CO, CO₂, NO, N₂O, NO₂, and H₂O. In short, the oxygen-containinggas means a gas of oxygen atoms or a gas of molecules containing oxygenatoms.

The atmospheric pressure in the heat treatment is preferably not lowerthan the pressure in which Group III nitride compound semiconductor isnot thermally decomposed at the temperature used for the heat treatmentWhen only O gas is used as the oxygen-containing gas, the gas may bepreferably fed with pressure not lower than the decomposition pressureof Group III nitride compound semiconductor. When O₂ gas is used whilemixed with another insert gas, all gases may be preferably fed withpressure not lower than the decomposition pressure of Group III nitridecompound semiconductor in the condition that the ratio of O₂ gas to allgases is not lower than about 10⁻⁶. In short, only a very small quantityof the oxygen-containing gas existing is sufficient. Incidentally, theupper limit of the quantity of the oxygen-containing gas fed is notparticularly limited from the point of view of p-type resistancereducing characteristic and electrode alloying characteristic. In short,the oxygen-containing gas can be used in a range allowing production.

The temperature for the heat treatment is most preferably in a range offrom 500 to 600° C. At a temperature of 500° C. or higher, there can beobtained a p-type Group III nitride compound semiconductor which is lowin resistance because of perfect saturation of resistivity. On the otherhand, at a temperature of 600° C. or lower, electrode alloying can beperformed well. The preferred temperature range is from 450 to 650° C.

For materials and heat treatment conditions for forming the padelectrode and the translucent electrode, refer to Japanese UnexaminedPatent Publication No. JP-A-H09-320984 and Japanese Unexamined PatentPublication No. JP-A-H10-209493.

In this specification, Group III nitride compound semiconductors arerepresented by the general formula: Al_(X)Ga_(Y)In_(1-X-Y)N (0≦X≦1,0≦Y≦1, 0≦X+Y≦1) which includes so-called binary compounds such as AlN,GaN and InN, and so-called ternary compounds such as Al_(X)Ga_(1-X)N,Al_(X)IN_(1-X)N and Ga_(X)In_(1-X)N (in the above, 0≦x≦1). The Group IIIelements may be partially replaced by boron (B), thallium (Ti), or thelike. The nitrogen (N) may be partially replaced by phosphorus (P),arsenic (As), antimony (Sb), bismuth (Bi), or the like. Each of theGroup III nitride compound semiconductors may contain an optionaldopant. Si, Ge, Se, Te, C, or the like, can be used as n-type impuritiesMg, Zn, Be, Ca, Sr, Ba, or the like, can be used as p-type impurities.Incidentally, the Group III nitride compound semiconductor doped withp-type impurities may be irradiated with electron beams or with plasmaor heated in a Furnace. The method for forming the Group III nitridecompound semiconductor layers is not particularly limited but the GroupIII nitride compound semiconductor layers may be formed by a metalorganic chemical vapor deposition method (MOCVD method) or may be formedby any other known method such as a molecular beam epitaxy method (MBEmethod), a halide vapor phase epitaxy method (HVPE method), a sputteringmethod, an ion-plating method, an electron showering method, etc.

Here, examples of the Group III nitride compound semiconductor deviceinclude: optical devices such as a light-emitting diode, alight-receiving diode, a laser diode, a solar cell, etc.; bipolardevices such as a rectifier, a thyristor, a transistor, etc.; unipolardevices such as an FET, etc.; and electronic devices such as a microwavedevice, etc. In addition, the present invention can be applied also tolaminates as intermediate products of these devices.

Incidentally, a homo structure, a hetero structure or a double heterostructure provided with a MIS junction, a PIN junction or a p-n junctionmay be used as the structure of the light-emitting device. A quantumwell structure (single quantum well structure or multiple quantum wellstructure) may be also used as the light-emitting layer.

An embodiment of the invention will be described below.

This embodiment is a light-emitting diode 80, the configuration of whichis shown in FIG. 7. Respective layers are as follows.

(Thick- Layer Composition Dopant ness) Translucent electrode 91 p-typeclad layer 85 p-GaN Mg  (0.3 μm) Light-emitting layer Superlattice 84structure Quantum well layer In_(0.15)Ga_(0.85)N  (3.5 nm) Barrier layerGaN  (3.5 nm) The number of iterations 1 to 10 of the quantum well layerand the barrier layer n-type clad layer 83 n-GaN Si  (4 μm) AlN bufferlayer 82 AlN  (60 nm) Substrate 81 sapphire (surface a) (300 μm)

The n-type clad layer 83 may be of a double-layered structure with an n−layer of a low electron density on the light-emitting layer 84 side andan n+ layer of a high electron density on the buffer layer 82 side. Thelatter is called n-type contact layer.

The light-emitting layer 84 is not limited to the superlatticestructure. A single or double hetero type structure, a homo-junctiontype structure, or the like, may be used as the structure of thelight-emitting device.

A layer of a Group III nitride compound semiconductor which has a wideband gap and which is doped with an acceptor such as magnesium may beinterposed between the light-emitting layer 84 and the p-type clad layer85. This is used for preventing electrons injected into the lightemitting layer 84 from diffusing into the p-type clad layer 85.

The p-type clad layer 85 may be of a double-layered structure with a p−layer of a low hole density on the light-emitting layer 84 side and a p+layer of a high hole density on the electrode side. The latter is calledp-type contact layer.

In the light-emitting diode configured in the aforementioned manner,each of the Group III nitride compound semiconductor layers is formed byexecution of MOCVD in a general condition.

Then, a mask is formed and the p-type clad layer 85, the active layer 84and the n-type clad layer 83 are partially removed by reactive ionetching to thereby reveal the n-type clad layer 83 on which the n-typepad electrode 94 is to be formed.

The translucent electrode material layer 91, the thick portion materiallayer 92 and the pad electrode material layers 93 and 94 are formed by alift off method.

A photo resist is applied on the semiconductor layer surface evenly andthen partially removed from an electrode-forming portion on the p-typeclad layer 85 by photolithography to thereby reveal this portion of thep-type clad layer 85. A Co layer (1.5 nm) and an Au layer (6.0 nm) arelaminated successively on the exposed p-type clad layer 85 by a vapordeposition apparatus to thereby form a translucent electrode maternallayer 91.

Then, a Co layer (3.0 nm) and an Au layer (6.0 nm) are laminatedsuccessively by the same lift-off method to thereby form a thick portionmaterial layer 92. Incidentally, the thick portion material layer 92 isshaped like a circular ring.

Then, a V layer (17.5 nm), an Au layer (1.5 μm) and an Al layer (10 nm)are laminated successively by the same lift-off method to thereby form ap-type pad electrode material layer 93. It is preferable thatindentation is formed in the circumferential edge of the opening portionof a mask used in this case.

An n-type pad electrode material layer 94 made of a combination ofvanadium and aluminum is formed in the same manner as described above.

The sample obtained in the aforementioned manner is put into a heatingfurnace. The furnace is evacuated to 1 Pa or less. Then, O₂ is suppliedinto the furnace up to ten and several Pa. In this state, the furnacetemperature is set at 55° C. and heat treatment is carried out for about4 minutes. Hence, as shown in FIGS. 1A, 1B, 3A and 3B, respectivematerials of the translucent electrode material layer 91, the thickportion material layer 92 and the pad electrode material layer 33 arealloyed and the layers are connected to one another to thereby form ap-type electrode structure. According to the inventors' examination,there is little current fed into the p-type clad layer just under thep-type pad electrode. It is supposed that this is because theaforementioned distribution inversion does not occur in the Au/Co vapordeposition layers constituting the translucent electrode just under thep-type pad electrode so that contact resistance becomes relatively high.Hence, the interface between the circumferential surface of the p-typepad electrode and the translucent electrode becomes a surface effectivein electrically connecting the two to each other. That is, the currentapplied in to the p-type pad electrode flows into the thick portion ofthe translucent electrode through the circumferential surface of thep-type pad electrode. The current is further diffused from the thickportion into the whole surface of the thin portion and fed into thewhole surface of the p type semiconductor layer evenly.

Then, a protective film of silicon dioxide is formed on the surface ofthe sample except bonding window portions by vapor deposition. Anyprotective film can be used if it has translucency and electricallyinsulating characteristic TiO₂, Al₂O₃, Si₃N₄, etc. may be used, besidesSiO₂. Alternatively, the protective film may be formed by sputtering orCVD.

The invention is not limited to the description of the mode for carryingout the invention and the embodiment thereof at all, but includesvarious modifications that can be easily conceived by those skilled inthe art, without departing from the claims for a patent.

1. A Group III nitride compound semiconductor device, comprising: atranslucent electrode comprising a thick portion and a residual portion,said residual portion comprising a thickness which is less than athickness of said thick portion, and said thick portion being formed ona circumferential surface of a p-type pad electrode.
 2. A Group mnitride compound semiconductor device according to claim 1, wherein saidthick portion of said translucent electrode comprises a materialdifferent from that of said residual portion of said translucentelectrode.
 3. A Group III nitride compound semiconductor deviceaccording to claim 1, wherein said translucent electrode comprises acobalt-gold alloy.
 4. A Group III nitride compound semiconductor deviceaccording to claim 2, wherein a thickness of said residual portion ofsaid translucent electrode is in a range of from 4 nm to 40 nm, andwherein a thickness of said thick portion is in a range of from 1.5times to 15 times the thickness of said residual portion.
 5. A Group IIInitride compound semiconductor device according to claim 4, wherein awidth of said thick portion is in a range of from 1 μm to 30 μm.
 6. AGroup III nitride compound semiconductor device according to claim 1,wherein said device has a structure of one of a light-emitting deviceand a light-receiving device.
 7. A translucent electrode in a Group IIInitride compound semiconductor device, comprising: a thick portion; anda residual portion having a thickness which is less than a thickness ofsaid thick portion, wherein said thick portion is formed on acircumferential surface of a p-type pad electrode.
 8. A translucentelectrode according to claim 7, wherein said thick portion comprises amaterial different from that of said residual portion of saidtranslucent electrode.
 9. A translucent electrode according to claim 7,wherein said thick portion comprises a cobalt-gold alloy.
 10. Atranslucent electrode according to claim 8, wherein a thickness of saidresidual portion is in a range of from 4 nm to 40 nm, and wherein athickness of said thick portion is in a range of from 1.5 times to 15times the thickness of said residual portion.
 11. A translucentelectrode according to claim 10, wherein a width of said thick portionis in a range of from 1 μm to 30 μm.
 12. A p-type electrode structure,comprising: a translucent electrode and a pad electrode formed on ap-type semiconductor layer in a Group III nitride compound semiconductordevice, wherein said translucent electrode comprises a thick portion anda residual portion said residual portion comprising a thickness which isless than a thickness of said thick portion, and said thick portionbeing formed on a circumferential surface of said pad electrode.
 13. Ap-type electrode structure according to claim 12, wherein said thickportion of said translucent electrode comprises a material differentfrom that of said residual portion of said translucent electrode.
 14. Ap-type electrode structure according to claim 12, wherein saidtranslucent electrode comprises a cobalt-gold alloy.
 15. A p-typeelectrode structure according to claim 13, wherein a thickness of saidresidual portion of said translucent electrode is in a range of from 4μm to 40 nm whereas a thickness of said thick portion is in a range offrom 1.5 times to 15 times the thickness of said residual portion.
 16. Ap-type electrode structure according to claim 12, wherein a width ofsaid thick portion is in a range of from 1 μm to 30 μm.
 17. A p-typeelectrode structure according to claim 12, wherein said circumferentialsurface of said pad electrode comprises an indented portion.
 18. A GroupIII nitride compound semiconductor device according to claim 1, whereinsaid residual portion is formed adjacent to said thick portion.
 19. AGroup III nitride compound semiconductor device according to claim 2,wherein said thick portion and said residual portion are continuouslyformed.
 20. A Group III nitride compound semiconductor device accordingto claim 1, wherein said thick portion of said translucent electrodecontacts a side surface of said p-type pad electrode.
 21. A p-typeelectrode structure according to claim 17, wherein said circumferentialsurface comprises a circumferential side surface, and wherein saidindented portion comprises one of a saw-toot shaped portion, awave-shaped portion and a hook-shaped portion.
 22. A Group III nitridecompound semiconductor device comprising: a pad electrode; and atranslucent electrode having a varying thickness and comprising a thickportion which is formed on a side surface of said pad electrode.
 23. Ap-type electrode structure, comprising: a pad portion; and a translucentportion having a varying thickness and comprising a thick portion whichis formed on a side surface of said pad portion.
 24. A Group III nitridecompound semiconductor device according to claim 1, wherein said p-typepad electrode is formed on said thick portion of said translucentelectrode and has an area which is smaller than an area of said thickportion.
 25. A Group III nitride compound semiconductor device accordingto claim 1, wherein an area of said thick portion of said translucentelectrode is within an area of said residual portion of said translucentelectrode.
 26. A Group m nitride compound semiconductor device accordingto claim 1, wherein said translucent electrode is formed around a bottomsidewall portion of said p-type pad electrode.
 27. A Group III nitridecompound semiconductor device according to claim 1, wherein saidtranslucent electrode and said p-type pad electrode comprise a goldalloy.
 28. A Group m nitride compound semiconductor device according toclaim 1, wherein said p-type pad electrode comprises a tapered sidewall,and said thick portion of said translucent electrode adjoins a bottom ofsaid tapered sidewall of said p-type pad electrode.
 29. A Group IIInitride compound semiconductor device according to claim 1, wherein acenter of said p-type pad electrode substantially coincides with acenter of said thick portion of said translucent electrode.
 30. A GroupIII nitride compound semiconductor device according to claim 1, whereinsaid thick portion of said translucent electrode has a substantiallyuniform width.
 31. A Group III nitride compound semiconductor deviceaccording to claim 1, wherein an interface between said translucentelectrode and said p-type pad electrode comprises an alloy of a materialof said translucent electrode and a material of said p-type padelectrode.
 32. A Group III nitride compound semiconductor deviceaccording to claim 1, wherein said thick portion of said translucentelectrode is formed around a periphery of said p-type pad electrode.